This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The discovery and development of new anti-infective drugs is urgently needed, as drug-resistant bacterial and viral diseases become an increasingly serious world health problem. The bacterial ribosome is a primary target of aminoglycoside antibiotics designed to combat bacterial infection. For instance, antibiotics such as paromomycin and tobramycin bind to the RNA aminoacyl decoding site (A-site) within the 16S ribosomal subunit and interfere with translation by inducing miscoding during the protein synthesis, which in turn leads to the death of the bacterial cell. The hepatitis C (HCV) internal ribosome entry site (IRES) element plays a central role in cap-independent translation of the viral genomic RNA. The unique conformation of IRES domain II is critical for 80S ribosomal assembly and initiation of viral translation. Here, the crystal structure of subdomain IIa of the HCV IRES has been determined at 2.3 A resolution, revealing the positions of divalent metal ions and complex inter-strand interactions that stabilize the L-shaped conformation of the RNA. The presence of divalent metal ions was necessary for crystal formation. Magnesium ions occupy specific sites that appear to be critical for the formation of the folded conformation. Subdomain IIa also was crystallized in the presence of strontium, which improved the diffraction quality of the crystals and the ability to identify interactions of the RNA with metal ions and tightly bound water molecules. The hinge region and noncanonical G-U base-pair motifs are stabilized by divalent metal ions and provide unique structural features that are potential interaction sites for small-molecule ligands. The information obtained from the crystal structure provides a basis for structure-guided design of HCV translation inhibitors targeting disruption of ribosomal assembly. This work was published in Acta Cryst. D.